On Monday, March 5, 2012 3:03:00 PM UTC, Graham. wrote:

The other thing Onetap might consider is that the resistor body is
physically in contact with strip (at least it is on this old Satchwell
I have on the desk), so the design was to heat the strip by conduction
primarily.

So the resistor is in contact with the bi-mettalic strip. The bi-metallic strip dissipates the heat to the air by convection and to the surroundings by radiation. The convection causes air currents to pass through the air vents. Anyone still having difficulty with that one? I don't think it's that hard to grasp.

The fact that the heater is only on when the load is has probably got something to do with the high thermal capacity of the metal and the high radiant emissivity of the metal surfaces. Metal surfaces are usually colder than the air temperature, and usually are the first to attract condensation. Then again, I didn't design the things and have never used one. Some control technicians thought the heater was effective for some applications..